Portable communication devices, such as cellular telephones, personal digital assistants (PDAs) and other communication devices often include multiple radio receivers or transceivers. For example, a cellular telephone may include a cellular transceiver, a television receiver and an FM radio. To minimize power consumption of an FM radio in handset applications, cell phone manufacturers have specified the use of a low frequency crystal, such as a 32.768 kHz crystal oscillator, as the external reference frequency source for the latest generation of FM radio integrated circuit (IC). The 32.768 kHz reference frequency must be multiplied many times to generate a local oscillator (LO) signal that enables the receiver to tune to channels in the FM frequency range of 76 to 108 MHz.
Implementing a low frequency reference source causes many design challenges. One of the challenges is that the frequency noise and frequency error of the low frequency (32.768 kHz in this example) reference are multiplied up by a large factor in the frequency synthesizer, resulting in large frequency error and frequency noise in the LO signal. For example, the multiplication factor for a 32.768 kHz reference source is about 3125 when the LO frequency is 102.4 MHz. The frequency error and frequency noise in the LO signal can degrade receiver selectivity and signal-to-noise-ratio (SNR), respectively, which are two key metrics of the performance of an FM receiver. It should be noted that in other radio frequency (RF) systems with higher LO frequencies, the problem will be worse if the same low frequency reference is used. For example, in a Bluetooth radio system where the LO frequency is about 2.48 GHz, the multiplication factor would be 75680 if a 32.768 kHz frequency reference source were used.
One way to avoid the large multiplication of frequency noise and error is to control the LO frequency through an automatic frequency control (AFC) circuit instead of a frequency synthesizer. While both a synthesizer and an AFC circuit have been used in the past, previous implementations of synthesizers and AFC circuits are analog intensive and require external components.
One previous analog methodology of implementing the synthesizer and the AFC circuit suffers from transients and spikes when switching from the synthesizer circuit to the AFC circuit, due to the analog nature of the circuits. Another drawback is that a large capacitance is used and cannot be easily integrated in the IC and therefore requires the use of external components.
Another previous analog methodology of implementing the frequency synthesizer and the AFC circuit uses a phase-and-frequency detector (PFD), a charge pump and a loop filter. A voltage controlled oscillator (VCO) tuning signal in the synthesizer and a VCO tuning signal in the AFC are both analog signals. The disadvantages of this implementation are that a large capacitance is used and cannot be easily integrated in the IC and therefore requires the use of external components.
Further, an analog adder may add more noise or consume more power, and, in the AFC mode even though the synthesizer does not affect the AFC operation as long as the frequency is within certain window, it nevertheless consumes extra power.
Therefore, it would be desirable to have a radio frequency (RF) tuning system and method that can be applied to an FM radio, or any RF receiver, that overcomes these challenges.